1. Field of the Invention
This invention relates to a fixating device for use in an image forming apparatus such as a copying apparatus or a printer, and particularly to a fixating device capable of heating a recording medium from its both sides.
2. Related Background Art
In recent years, the production of color copying apparatuses of the electrophotographic type has been done actively, and since this type is excellent in image quality and running costs as compared with the other types, its market has widened. Further, the application of this type to color printers is going on and the age of color DTP is about to spread. Accordingly, in a fixating device for color used in such an image forming apparatus, it is regarded as being necessary to improve the mixability of colors and OHP transmissivity in terms of image quality.
So, in order to heat both of toners and paper equally and sufficiently melt and mix thick toner images of multiple colors superposed one upon another, there has been proposed a construction in which heaters as heat sources are provided in both of a fixating roller and a pressing roller. Also, with regard to temperature control, it is possible to bring a thermistor into contact with the surface of each roller and detect the surface temperature of each roller to thereby effect temperature control, but such control becomes complicated in itself and leads to an increased cost and therefore, it is preferable to bring the thermistor into contact with only the surface of one of the two rollers and detect the surface temperature thereof to thereby effect temperature control.
This system, however, has suffered from the following disadvantages. First, from the viewpoints of the stability of temperature control and fixative property, it is desirable that the temperature difference between the upper and lower rollers be within the order of 10.degree. C., and to make the rising characteristics of the upper and lower rollers equal to each other and suppress overshoot, it is necessary to make the ratios of heater rated output to roller heat capacity equal for the respective ones of the upper and lower rollers.
However, even in such a construction wherein the rising characteristics are uniformized, there has been the problem that when the upper and lower rollers are left stopped upon standby, a heat difference is created between the upper and lower rollers by the amount of escape of heat, i.e., the so-called heat leak, of the upper and lower rollers. One of the causes of the creation of such heat difference has been that as shown in FIG. 12 of the accompanying drawings, a hot air stream flows from the lower roller 2 side to the upper roller 1 side, whereby a temperature gradient which becomes upwardly higher is formed in the fixating unit itself.
Accordingly, in a system wherein the thermistor is brought into contact with the lower roller to effect temperature control, the upper roller side is smaller in heat leak and more liable to rise in temperature than the lower roller side, and this has led to the disadvantage that the life and safety of the roller are low. Also, in a system wherein the thermistor is brought into contact with the upper roller, the lower roller side becomes liable to fall in temperature and unless pre-rotation is effected long before printing, the temperatures of the upper and lower rollers will not be stable.
Further, when the heat leak difference between the upper and lower rollers is measured and the rated electric power of the upper roller is set to a value lower than the rated electric power of the lower roller to thereby make the temperature difference between the upper and lower rollers smaller, the temperature difference becomes null during standby and attains an effect, but conversely a temperature difference is given birth during continuous print output.